Doc: Capitalize header file in code snippet

Fixes: QTBUG-113584
Pick-to: 6.5 6.4 6.2
Change-Id: I2910eba4710e527baf258176921774dfa16b1485
Reviewed-by: Amir Masoud Abdol <[email protected]>
Reviewed-by: Ulf Hermann <[email protected]>

Author: andreaseliasson

src/qml/doc/src/qtqml-writing-a-module.qdoc

@@ -92,7 +92,7 @@ You can use the \l Q_IMPORT_QML_PLUGIN macro to create a reference to this symbo
 Add the following code to the main.cpp:
 
 \badcode
-#include <QtQml/qqmlextensionplugin.h>
+#include <QtQml/QQmlExtensionPlugin>
 Q_IMPORT_QML_PLUGIN(ExtraModulePlugin)
 \endcode